Isoroku Yamamoto (山本 五十六, Yamamoto Isoroku, April 4, 1884 – April 18, 1943) was a Japanese Marshal Admiral of the Navy and the commander-in-chief of the Combined Fleet during World War II until his death.

After graduating from the Imperial Japanese Naval Academy in 1904, Yamamoto served on the armored cruiser Nisshin during the Russo-Japanese War. He was wounded at the Battle of Tsushima, losing two fingers (the index and middle fingers) on his left hand, as the cruiser was hit repeatedly by the Russian battle line. He returned to the Naval Staff College in 1914, emerging as a lieutenant commander in 1916.

In 1916, Isoroku was adopted into the Yamamoto family (another family of former Nagaoka samurai) and took the Yamamoto name. It was a Common practice for samurai families lacking sons to adopt suitable young men in this fashion to carry on the family name, the rank and the income that comes with it. In 1918 Isoroku married Reiko Mihashi, with whom he had two sons and two daughters.

Yamamoto was part of the Japanese Navy establishment, who were rivals of the more aggressive army establishment, especially the officers of the Kwantung Army. As such, he promoted a policy of a strong fleet to project force through gunboat diplomacy, rather than a fleet used primarily for transport of invasion land forces, as some of his political opponents in the army wanted. This stance led him to oppose the invasion of China. He also opposed war against the United States partly because of his studies at Harvard University (1919–1921) and his two postings as a naval attaché in Washington, D.C., where he learned to speak fluent English. Yamamoto traveled extensively in the United States during his tour of duty there, where he studied American customs and Business practices.

He was promoted to captain in 1923. On February 13, 1924, at the rank of captain, he was part of the Japanese delegation visiting the US Naval War College. Later that year, he changed his specialty from gunnery to naval aviation. His first command was the cruiser Isuzu in 1928, followed by the aircraft carrier Akagi.

He participated in the second London Naval Conference of 1930 as a rear admiral and the 1934 London Naval Conference as a vice admiral, as the growing military influence on the government at the time deemed that a career military specialist needed to accompany the diplomats to the arms limitations talks. Yamamoto was a strong proponent of naval aviation, and served as head of the Aeronautics Department before accepting a post as commander of the First Carrier Division. Yamamoto opposed the invasion of Manchuria in 1931, the subsequent land war with China (1937), and the 1940 Tripartite Pact with Nazi Germany and fascist Italy. As Deputy Navy Minister, he apologized to United States Ambassador Joseph C. Grew for the bombing of the gunboat USS Panay in December 1937. These issues made him a target of assassination threats by pro-war militarists.

As a strategic blow intended to prevent American interference in the Dutch East Indies for six months, the Pearl Harbor attack was a success, but unbeknownst to Yamamoto, it was a pointless one. In 1935, in keeping with the evolution of War Plan Orange, the US Navy had abandoned any intention of attempting to charge across the Pacific towards the Philippines at the outset of a war with Japan. In 1937, the US Navy had further determined even fully manning the fleet to wartime levels could not be accomplished in less than six months, and myriad other logistic assets needed to execute a trans-Pacific movement simply did not exist and would require two years to construct after the onset of war. In 1940, US Chief of Naval Operations, Admiral Harold Stark had penned a Plan Dog memorandum, which emphasized a defensive war in the Pacific while the US concentrated on defeating Nazi Germany first, and consigned Admiral Husband Kimmel's Pacific Fleet to merely keeping the Imperial Japanese Navy (IJN) out of the eastern Pacific and away from the shipping lanes to Australia. Moreover, it is questionable whether the US would have gone to war at all had Japan attacked only British and Dutch possessions in the Far East.

Throughout 1938, many young army and naval officers began to speak publicly against Yamamoto and certain other Japanese admirals such as Mitsumasa Yonai and Shigeyoshi Inoue for their strong opposition to a tripartite pact with Nazi Germany as they saw it as inimical to "Japan's natural interests." Yamamoto received a steady stream of hate mail and death threats from Japanese nationalists. His reaction to the prospect of death by assassination was passive and accepting. The admiral wrote: To die for Emperor and Nation is the highest hope of a military man. After a brave hard fight the blossoms are scattered on the fighting field. But if a person wants to take a life instead, still the fighting man will go to eternity for Emperor and country. One man's life or death is a matter of no importance. All that matters is the Empire. As Confucius said, "They may crush cinnabar, yet they do not take away its color; one may burn a fragrant herb, yet it will not destroy the scent." They may destroy my body, yet they will not take away my will.

The Japanese Army, annoyed at Yamamoto's unflinching opposition to a Rome-Berlin-Tokyo treaty, dispatched military police to "guard" Yamamoto, a ruse by the army to keep an eye on him. He was later reassigned from the naval ministry to sea as the commander-in-chief of the Combined Fleet on August 30, 1939. This was done as one of the last acts of the then-acting Navy Minister Mitsumasa Yonai, under Baron Hiranuma's short-lived administration. It was done partly to make it harder for assassins to target Yamamoto. Yonai was certain that if Yamamoto remained ashore, he would be killed before the year [1939] ended.

As Japan moved toward war during 1940, Yamamoto gradually moved toward strategic as well as tactical innovation, again with mixed results. Prompted by talented young officers such as Lieutenant Commander Minoru Genda, Yamamoto approved the reorganization of Japanese carrier forces into the First Air Fleet, a consolidated striking force that gathered Japan's six largest carriers into one unit. This innovation gave great striking capacity, but also concentrated the vulnerable carriers into a compact target. Yamamoto also oversaw the organization of a similar large land-based organization in the 11th Air Fleet, which would later use the G3M and G4M to neutralize American air forces in the Philippines and sink the British "Force Z".

On the strategic level, the attack was a disaster for Japan, rousing American passions for revenge due to it being a "sneak attack". The shock of the attack coming in an unexpected place, with such devastating results and without the expected "fair play" of a declaration of war galvanized the US public's determination to avenge the attack. When asked by Prime Minister Fumimaro Konoe in mid-1941 about the outcome of a possible war with the United States, Yamamoto made a well-known and prophetic statement: If ordered to fight, "I shall run wild considerably for the first six months or a year, but I have utterly no confidence for the second and third years." His prediction would be vindicated as Japan easily conquered territories and islands for the first six months of the war until it suffered a shattering defeat at the Battle of Midway on June 4–7, 1942, which ultimately tilted the balance of power in the Pacific towards the US.

To prevent the Japanese from regaining the initiative, King ramrodded the idea of an immediate American counterattack through the Joint Chiefs of Staff. This precipitated the US invasion of Guadalcanal and beat the Japanese to the punch, with marines landing on the island in August 1942 and starting a bitter struggle that lasted until February 1943 in a battle of attrition that Japan could ill afford.

Yamamoto's staff cremated his remains at Buin and his ashes were returned to Tokyo aboard the battleship Musashi, Yamamoto's last flagship. Yamamoto was given a full state funeral on June 5, 1943, where he received, posthumously, the title of Marshal and was awarded the Order of the Chrysanthemum (1st Class). He was also awarded Nazi Germany's Knight's Cross of the Iron Cross with Oak Leaves and Swords. Some of his ashes were buried in the public Tama Cemetery, Tokyo (多摩霊園) and the remainder at his ancestral burial grounds at the temple of Chuko-ji in Nagaoka City. He was succeeded as commander-in-chief of the Combined Fleet by Admiral Mineichi Koga.

The first film to feature Yamamoto was Toho's 1953 film Taiheiyô no washi, (later released in the United States as Eagle of the Pacific), in which Yamamoto was portrayed by Denjirô Ôkôchi.

Yamamoto practiced calligraphy. He and his wife, Reiko, had four children: two sons and two daughters. Yamamoto was an avid gambler, enjoying Go, shogi, billiards, bridge, mah jong, poker, and other games that tested his wits and sharpened his mind. He frequently made jokes about moving to Monaco and starting his own casino. He enjoyed the company of geisha, and his wife Reiko revealed to the Japanese public in 1954 that Yamamoto was closer to his favorite geisha Kawai Chiyoko than to her, which stirred some controversy. After his death, his funeral procession passed by Kawai's quarters on the way to the cemetery.

The 1960 film The Gallant Hours depicts the battle of wits between Vice-Admiral william Halsey, Jr. and Yamamoto from the start of the Guadalcanal Campaign in August 1942 to Yamamoto's death in April 1943. The film, however, portrays Yamamoto's death as occurring in November 1942, the day after the Naval Battle of Guadalcanal, and the P-38 aircraft that killed him as coming from Guadalcanal.

In Daiei Studios's 1969 film Aa, kaigun (later released in the United States as Gateway to Glory), Yamamoto was portrayed by Shôgo Shimada.

One of the most notable films is the 1970 movie Tora! Tora! Tora!, which stars Japanese actor Sô Yamamura as Yamamoto, who states after the attack on Pearl Harbor:

In Shūe Matsubayashi's 1981 film Rengō kantai (lit. "Combined Fleet", later released in the United States as The Imperial Navy), Yamamoto was portrayed by Keiju Kobayashi.

In the 1993 OVA series Konpeki no Kantai (lit. Deep Blue Fleet), instead of dying in the plane crash, Yamamoto blacks out and suddenly wakes up as his younger self, Isoroku Takano, after the Battle of Tsushima in 1905. His memory from the original timeline intact, Yamamoto uses his knowledge of the Future to help Japan become a stronger military power, eventually launching a coup d'état against Hideki Tōjō's government. In the subsequent Pacific War, Japan's technologically advanced navy decisively defeats the United States, and grants all of the former European and American colonies in Asia full independence. Later on, Yamamoto convinces Japan to join forces with the United States and Britain to defeat Nazi Germany.

In Neal Stephenson's 1999 book Cryptonomicon, Yamamoto's final moments are depicted, with him realising Japan's naval codes have been broken and he must inform headquarters.

In the 2001 film Pearl Harbor, Yamamoto was portrayed by Oscar-nominated Japanese-born American actor Mako Iwamatsu. Like Tora! Tora! Tora!, this film also features the sleeping giant quote.

In the 2004 anime series Zipang, Yamamoto (voiced by Bunmei Tobayama) works to develop the uneasy partnership with the crew of the JMSDF Mirai, which has been transported back sixty years through time to the year 1942.

In Douglas Niles' 2007 book MacArthur's War: A Novel of the Invasion of Japan (written with Michael Dobson), which focuses on General Douglas MacArthur and an alternate history of the Pacific War (following a considerably different outcome of the Battle of Midway), Yamamoto is portrayed sympathetically, with much of the action in the Japanese government seen through his eyes, though he could not change the major decisions of Japan in World War II.

In Robert Conroy's 2011 book Rising Sun, Yamamoto directs the IJN to launch a series of attacks on the American West Coast, in the hope the United States can be convinced to sue for peace and securing Japan's place as a world power; but cannot escape his lingering fear the war will ultimately doom Japan.

The plan was a compromise and hastily prepared, apparently so it could be launched in time for the anniversary of Tsushima, but appeared well thought out, well organized, and finely timed when viewed from a Japanese viewpoint. Against four carriers, two light carriers, 11 battleships, 16 cruisers and 46 destroyers likely to be in the area of the main battle the US could field only three carriers, eight cruisers, and 15 destroyers. The disparity appeared crushing. Only in numbers of carrier decks, available aircraft, and submarines was there near parity between the two sides. Despite various mishaps developed in the execution, it appeared that—barring something unforeseen—Yamamoto held all the cards.